The present invention relates to a method of controlling the shape of a rolled sheet in a tandem type rolling mill.
Usually, a rolled sheet, especially a thin sheet is prepared by rolling it in a rolling mill. In this method, the sheet is rolled to elongate it in the longitudinal direction so as to form a thinner sheet.
The elongation is dependant upon the ratio of draft to [(sheet thickness at input side--sheet thickness at output side)/(sheet thickness at input side)]. The distribution of the elongation in the transversal direction is dependant upon the distribution of the sheet thickness at the input side in the transversal direction and upon the distribution of the sheet thickness in the transversal direction after the rolling.
The distribution of the sheet thickness after the rolling is affected by the deformation of the rolling rolls such as:
1. an elastic deformation of the rolling roll; PA1 2. a thermal expansion of the rolling roll caused by conducting heat from the rolled sheet to the rolling rolls; and PA1 3. a wear of the rolling rolls caused by friction between the rolled sheet and the rolling roll.
The distribution of the elongation in the transversal direction is caused by the elongation of the rolled sheet in the longitudinal direction wherein compressing stress and tensile stress in the longitudinal direction remain as the distribution in the transversal direction. When the stresses are higher than certain limits, deformation of the rolled sheet is produced resulting in a backing phenomenon which is called a shape defect.
FIGS. 1, 2, and 3 show the relationship between the distribution of the sheet thickness after the rolling and the shape defect of the rolled sheet caused by the distribution of the sheet thickness for the case of a constant sheet thickness the input side.
In FIGS. 2 and 3, FIGS. 1A, 2A, and 3A show a schematic view of a rolled sheet having shape defect; FIGS. 1B, 2B, and 3B show a sectional view of the rolled sheet in the transversal direction; FIGS. 1C, 2C, and 3C show a tension distribution of the rolled sheet in the transversal direction; and FIGS. 1D, 2D, and 3D show a distribution of the rolled sheet in the transversal direction.
The shape defect shown in FIG. 1 is called a middle elongation or a center backing.
The condition shown in FIG. 2 is called a lug wave or a wave edge. This shape defect causes a failure of the apparatus or a deterioration of quality in the later steps.
Heretofore, in order to prevent such shape defects in the rolled sheet, the roll bending force under the last stand has been controlled. That is, in the conventional shape controlling method, it has been considered to be optimum to provide uniform front tension at the last stand. However, in the tandem type rolling mill, the uniformity of the draft distribution at the first stand in the transversal direction is usually difficult to maintain in that the distribution of the speed at the output side of the first stand is not uniform and has a certain distribution in the transversal direction. Thus, the backward slip at the first stand (provided one does not consider the back tension at the first stand) in the transversal direction is not uniform and is varied depending upon the shape of the sheet and the distribution of the sheet thickness at the input side of the first stand.
In order to produce a uniform distribution of the backward slip, it is necessary to provide a certain tension distribution between the pay-off reel and the first stand. Accordingly, even though a uniform front tension at the last stand is supplied, it has been impossible to produce a desired shape of the rolled sheet because of the back tension distribution at the first stand.